1. Field of the Invention
This invention relates to a wireless local area network, and, more particularly, to a wireless local area network including a stationary access point and a plurality of mobile wireless devices, in which it is desirable to increase the maximum allowable distance for transmission between the stationary access point and one or more of the mobile wireless devices.
2. Summary of the Background Art
In a number of locations, a wireless local area network (WLAN) is used to provide one or more wireless mobile units (MUs), such as portable computing systems having short-range radio transmission capabilities, with an ability to connect to a conventional wired local area network (LAN) through a stationary access point (AP) connected to the LAN. In particular, increasing numbers of such WLANs are built with devices conforming to the IEEE 802.11 standard, which provides the MUs with abilities to connect to one another, to move around within an area of coverage allowing communication with a single AP, and to seamlessly move from a area in which a connection is made with one AP to an area in which a connection is made with another AP.
FIG. 1 is a block diagram of a number of MUs 10 connected to form a conventional ad-hoc network 12 in accordance with the IEEE 802.11 standard. All of the MUs 10 are connected directly to one another by radio links 14. Since no AP is present within the ad-hoc network 12, the MUs 10 can only communicate with one another, and therefore form an independent basic service set (BSS). There is no way to communicate with the rest of the world. Such a network 12 is formed when a number of individuals, wishing to share data and having MUs operating according to the IEEE 802.11 standard, meet in a location, such as a conference room, in the absence of an AP. The ad-hoc network 12 may be formed in an automatic fashion as the operating MUs 10 are brought close enough to one another to begin communications while determining that an AP is not present.
FIG. 2 is a block diagram of a number of MUs 10 connected to form a pair of conventional infrastructure basic service sets (BSSs) 16, 17 also in accordance with the IEEE 802.11 standard, which defines media access control (MAC) layer and physical (PHY) layer specifications for a wireless LAN. The first infrastructure BSS 16 includes an AP 18, which is connected by wires to a conventional wired LAN 20. The second infrastructure BSS 17 includes an AP 19, which is also connected by wires to the LAN 20. Each infrastructure BSS 16, 17 is limited to a number of MUs 10 that are in range for communication with the AP 18, 19. Within each infrastructure BSS 16, 17 each operating MU 10 is connected directly to the AP 18 by means of a radio link 22; the MUs 10 are not connected directly to one another. An individual MU 10 can use this system to communicate with another MU 10 in the same infrastructure BSS 16, with an MU 10 in another infrastructure BSS 17 connected to the LAN 20, or for various purposes conventionally achieved through connection to a LAN, such as obtaining access to the Internet.
For a message to be transmitted to or from an MU 10 within a BSS 16, the MU 10 must be associated with the AP 18 within the BSS 16. The process of association, which synchronizes the MU 10 with the AP 18 for communication, is begun by the MU 10 using an association service of the AP 18. According to the IEEE 802.11 standard, the MU 10 begins the association process by scanning to determine which APs 18 can be reached from the location of the MU 10 and by requesting association with a single AP 18. The MU 10 may use a passive scanning process, monitoring beacon frames transmitted by the APs 18 to determine which AP 18 is close enough for communications. Alternately, the MU 10 may use an active scanning process, transmitting probe frames. An AP 18 close enough to receive the probe frames then transmits probe response frames if certain criteria are met by the probe frames.
An important feature of the deployment of WLANs according to the IEEE 802.11 standard is the provision for an extended service set (ESS) architecture, in which a number of APs 18, 19 communicate with one another to forward data traffic from one BSS 16 to another BSS 17, and to switch a roaming MU 10 from one BSS 16 to another BSS 17. These switching functions are performed by the distribution system (DS), serving as the spine of the WLAN.
In a WLAN operating according to IEEE 802.11, the AP 18 provides an authentication service, which, in defining the identity of a particular MU 10, can be used to determine whether the MU 10 is allowed access to the LAN 20 by comparing the Media Access Control (MAC) address of the MU 10 with a list of acceptable addresses stored within the AP 18 or at another location accessible through the LAN 20. The MAC address is a hardware-level machine address code given to the MU 10 or to a circuit element within the MU 10 at its time of manufacture. Every MAC address is unique, so no two MUs can have the same MAC address. For example, if the MU 10 is a portable personal computer communicating through a network interface card (NIC) built in a PC Card format for establishing wireless communications a MAC address stored in non-volatile storage within the NIC at its time of manufacture is the MAC address of the MU 10.
To facilitate operation within the BSS architecture, the MU 10 may cause itself to be authenticated with additional APs 18 in adjacent BSSs 16. While an MU 10 can be associated with only one AP 18 at a time, it can be authenticated by a number of APs 18, 19. In order to free resources of the AP 18 for use with other MUs 10, the AP 18 also performs a de-authentication service, eliminating a previously known station identity, when the MU 10 shuts down or when it roams out of the range of the AP 18.
The AP 18 can also perform a disassociation service, eliminating its association with the MU 10 when the MU 10 roams out of range, when the AP 18 is shutting down, or for a number of other reasons. When this occurs, the MU 10 must use the association service of the WLAN to connect to another AP 19.
A particularly important feature of a WLAN built in accordance with the IEEE 802.11 standard is the ability given the user of an MU 10 to roam from one BSS to another, for example, within an office building, within a home, or on a college campus, without a need to modify network services. In an environment built to provide for such roaming, the overlapping area 21 between adjacent BSSs is substantial to allow for switching between one AP 18 another AP 19. To avoid interference, the adjacent APs 18, 19 are assigned different frequency channels among the eleven channels provided under the IEEE 802.11 standard.
This roaming capability also results from an ability of the MU 10 to determine the quality of a signal from each AP 18, 19 in range and to determine when to switch to from an AP 18 to another AP 19, from which a stronger or cleaner signal is received, as determined by the signal-to-noise (S/N) ratio of the signal. Even when an MU 10 is associated with an AP 18, the MU 10 monitors the beacon frames transmitted by other APs 19. These beacon frames contain link measurement data and information describing the transmitting AP 19. When a comparison of S/N ratios indicates that a switch should be made, the MU 10 transmits authentication information and attempts the reassociate with the new AP 19.
A reassociation service requested by the MU 10 and provided by the new AP 19 provides for changing the association with the MU 10 from one AP 18 to another AP 19, without a requirement, as the term might be construed to imply, that the MU 10 had previously been associated with the new AP 19. In the process of reassociation, the MU 10 transmits information telling the new AP 19 the identity of the old AP 18, from which the switch is being made. Then, the new AP 19 gets ANY data frames left at the old AP 18 and notifies the old AP 18 not to accept messages for the MU 10.
Because of the complex characteristics of radio transmission in many environments, and because of the fluid nature of a BSS 16 which MUs 10 can constantly enter, leave, and request various services, changing loading conditions of the AP 18, the process of designing a WLAN to reliably transmit messages under foreseeable conditions is difficult. Under ideal conditions, a single AP 18 of a commercially available type can communicate with up to 128 MUs 10 at distances up to 457 m (1500 ft). Under actual conditions in commercial buildings APs 18, 19 may need to be spaced to provide maximum operating distances of only 15.2 to 30.5 m (50 to 100 ft). The placement of APs 18, 19 and the types of radio antennas to use with them, which may be omnidirectional or directional, is also determined by sources of interference, such as microwaves ovens, cellular phones, mechanical rooms for air conditioning units, other communications equipment, and elevators.
Due to such complexities, an actual operating WLAN environment may include gaps in coverage by the APs 18. This is particularly true if one of the APs 18 cannot be accessed by an MU 10 because the AP 18 has failed or become overloaded with other communications. Furthermore, it may be possible that the entire possible WLAN environment is not covered by APs due to budget constraints. What is needed is a method for an MU 10 outside all of the infrastructure BSSs 16, 17 to be able to access the AP 18, 19 in one of the infrastructure BSSs 16, 17.
U.S. Pat. Nos. 5,884,031 and 6,249,810 describe methods for connecting client devices in a wired network to receive information and also to retransmit the information to other client devices, so that information can be broadcast from a single server or Internet transmitter to a number of client devices much greater than the number of such devices that can be directly connected to the server or Internet transmitter itself.
In the method of U.S. Pat. No. 5,884,031, a pre-determined number of client systems are first allowed to connect directly to a server system. After this occurs, the server furnishes additional client systems requesting connection with the addresses of client systems already connected to form a private network. Each of the client systems then makes connections with a multiple number of client systems to receive information from the server system. Each of these client systems subsequently accepts connections from up to a second predetermined number of client systems to which it transmits information received from the server system.
In the method of U.S. Pat. No. 6,249,810, a client system, operating as a “radio device” and employing a specialized graphical user interface, receives a list of Internet “radio station” transmitters. To hear a broadcast, the user selects a station from this list, causing the client system to contact a transmission scheduler connected to the Internet. The transmission scheduler causes the client system to be connected in a chain, generally to receive information retransmitted from another client system. The transmission scheduler supervises these connections, making new connections as needed when client systems sign off.
What is needed is a method for connecting client systems by radio links to achieve access to a access point, without a need to first access a central point, such as the server system of U.S. Pat. No. 5,884,031 or the transmission scheduler of U.S. Pat. No. 6,249,810.